What Spring Rates To Use In Your Race Car - Spring Technology Explained

Springs are one of the primary components we use to set up our car. We are fortunate in circle track racing that we only turn left. So, we can manipulate the spring rates on the four corners to whatever will suit the setup. When we have chosen the correct combination of spring rates to go along with the other parameters, we get what we want-a balanced and fast race car.

2/13The basic rates of the springs we use in circle track racing have changed a lot over the past few years. The labeled spring rates mean little unless we know something about the suspension systems in our race cars. Teams have found many varied ways to crutch the spring setups in their cars. Installing quality springs and knowing the true rates is just the beginning of understanding the effects of spring rates on the setup in your car.

In order to win races, our car must be fast, not only right out of the box, but at the end of the race too. In fact, many races are won by cars that may not be the fastest car in the early laps of the race, but prove to be faster than the field at the end of the race. The key is to make sure we give the car what it wants so the car will be fast as well as consistent.

In a short sprint race of 25 or 30 laps, a jackrabbit setup may work relatively well. A team with a well-balanced setup that may start off a couple of tenths slower for the first 10-15 laps may not have enough time to take advantage of everyone else's lap time falling off as the laps count down. But in a 50- or 100-lap race, the car that falls off less as the laps move past lap 25 or so will have a much better opportunity to win. A car that is closer to balanced will have the best chance to win in any length race.

The Balanced Setup The key to chassis performance, whether you are running on dirt or asphalt, is to work toward a more balanced setup. This can be done by trial and error (by far the most commonly used method) or by giving the car what it wants. In the past, trial and error took a lot of time and the teams really had no idea when the proper balance was achieved. We now have discovered indicators that we can use to help determine when we have truly achieved a balanced setup.

3/13A spring rate's influence on the setup is dictated by the installation method. In a Formula car, we see a similar situation in that there is a motion ratio. The wheel rate is much different than the spring rate. In this installation, a push rod extends from the lower control arm to a rocker arm that is connected to the coilover spring. The spring moves much less distance for every movement of the push rod, which is at an extreme angle to the control arm. In a Stock Car there are motion ratios too.

Many odd arrangements of springs have been tried and right now we are seeing some racers across the country trying new methods to find a balance. For every car there may be dozens of combinations of spring rates, moment center locations front and rear, and weight combinations that will balance the car. Here are some considerations concerning springs when trying to balance your race car.

Knowing the True Spring Rate The very first thing to consider when talking about what rate springs to use in your race car is knowing exactly what the spring rate is for each of your springs. You must test each spring and do it in the proper way. We need to know the spring rate of our springs at the range that they will be working in conjunction with the corner where it will be working.

The RF spring is mostly in compression, or bump, so we would compress that spring to the installed height and then further compress it 2-4 inches to find the average rate within the range. Remember that there is an installed motion ratio and the spring is moving less than the wheel, so we don't necessarily need to compress the spring as much as the wheel moves.

4/13It's a good idea to buy or borrow a good quality spring rating device so you can accurately determine the installed spring rate of each spring you might use. Compress the spring to the installed height and then rate the spring from there. Here we see a team measuring spring rate with the gas pressure shock installed to learn the combined spring rate that includes the coil and the shock spring rate caused by the pressure. Theoretically, we should rate a spring based on which corner of the car it will be used on because of the different ranges of motion. For the LF spring we would measure the height of the installed spring and compress the spring in the spring rating device to that exact height before rating. Then, for a conventional setup, this corner might bump and rebound up to an inch or more. Check the rate up and down 2 inches from the installed height. For softer spring setups, this corner might travel up to 3 inches or more in compression.

The RR spring is also mostly in compression and would be compressed initially to ride height and then up to 3-4 inches further, depending on the spring rate. For big bar and soft spring setups on asphalt, the right rear spring is much stiffer and travels much less than a conventional rate spring.

The LR spring reacts similarly to the LF spring in that it moves less compared to the springs on the right side of the car. It's in some rebound on entry and some compression at mid-turn. This spring would be rated by compressing to ride height and then moving up to 2 inches up and down. For dirt car applications, this spring may rebound quite a bit as the left side jacks up for some slick track setups. In this case, the spring may well be very near or past the free (unloaded) height at mid-turn and compressed down the straightaway.

Front Spring Split The overall trend in circle track racing, for both dirt and asphalt racing, has been to reduce the rates of the front springs. There is even a move for some situations, to a softer right front spring.

Dirt cars can benefit from a softer right front spring on flatter dry and slick tracks. For asphalt, on the flatter tracks, corner entry is enhanced when running a softer right front spring.

5/13The wheel rates for most coilover designs is affected by both the motion ratio and the spring angle. Both the angle and installation ratio effects are squared to find the wheel rate.

For high banked tracks, the front spring rate must be increased and it is often necessary to stiffen the right front spring more so than the left front spring rate.

No matter what the stiffness is for our front springs, we still need to compensate at the rear in order to balance the car's suspension dynamics. Dirt Late Model teams are running much stiffer right rear springs than ever before and seeing a lot of success. Asphalt teams who are trying the BBSS setups tend to overdo the stiffening of the RR spring.

On flatter tracks, we have seen where, with some rough tracks, the very stiff RR spring just does not work and will upset the car due to excess bouncing going over the ripples or bumps. There is a limit to how stiff a corner can be.

6/13Coil spring (stock big spring) cars are only affected by the motion ratio because the spring moves very closely to perpendicular to the motion of the arm. We still need to make sure that the supports at the ends of the springs are flat and perpendicular to the centerline of the spring when the spring is compressed after dive and roll.

On the higher banked racetracks, teams that try the soft front springs will quickly change to a higher rate when the car bottoms out. If we use our common sense, we will know that the added downforce will overcome the light spring rate and either the springs will go into coil bind and/or the chassis, and hopefully not the oil pan, will contact the track surface first.

Rear Spring Rate Split When racing on flatter tracks, we can often run a softer RR spring rate so we can help increase the amount of bite off the corners. Some teams running the conventional setups will go to extremes with this spring split and end up with 25 to 50 pounds of spring rate difference. Most of the time we do not need a high amount of spring split to achieve the desired effect. I have personally set up winning cars in major series on flat tracks while using only 10-15 pounds of rear spring split.

It's very hard to balance the front and rear suspension systems when using a high amount of rear spring split, so we try to go with as little as possible when trying to gain bite off the corners on flatter tracks.

7/13Be sure to measure your control arms correctly. We need to have accurate distances for the position of the spring in relation to the rotational radius of the control arm. The rotational radius is the shortest (at 90 degrees) distance from the ball joint to the rotational axis of the inner mounts of the control arm.

With conventional setups on the higher banked tracks of more than 12 degrees, we can run a rear spring split with the RR spring rate higher than the LR spring rate to help balance the front and rear suspensions. This helps to control the rear roll of the car so that we do not need to raise the Panhard bar to excessively high levels.

Improper Switch Between Installation Ratios It's common for a team to buy a new car from a different manufacturer or move to a new class where the cars are constructed differently than what we are used to. What often changes is the installation ratio at the front of the car and spring base at the rear.

Suppose we have run a class using stock spring rates at the front with stock lower control arms and now decide to run a class that uses a fabricated front clip and coilover shocks and springs that are mounted differently on the lower control arms. If we had the setup figured out on the old car as far as wheel rates were concerned, we then need to duplicate those wheel rates (assuming the overall car weights remain about the same) in order to stay on track with our handling.

To do this we need to know how to calculate the wheel rates in each car. We first work out the old wheel rates and then try different springs in the calculations until the new car has the same wheel rates. If we are going from a perimeter car (symmetric from left to right sides) to an offset chassis, the problem is compounded because the lower control arms are different lengths.

8/13For a solid axle rear suspension, the rear spring base is felt by the car at the top of the springs. The wider the tops of the springs are mounted from each other, the less roll tendency the rear suspension will have. Narrow spring bases promote suspension roll and that may be detrimental to achieving a balanced setup.

Wheel rate is determined by using the installed ratio (the position of the spring on the control arm), as well as the angle of the spring in relation to the line between the center of rotation of the ball joint and the inner pivots of the control arm. The motion ratio is the distance from the inner pivots to the center of the spring divided by the length of the control arm.

We need to square that number. The "installed ratio squared" number is then multiplied by the square of the Cosine function of the spring angle measured from vertical, and that answer is multiplied times the spring rate in pounds per inch. The result is the wheel rate at that corner of the car.

9/13In an offset chassis, even if we install the same spring rates on both sides in the front, we will have different wheel rates due to the different motion ratios at each wheel. This is because the lower control arms are different lengths. This formula will calculate the wheel rate for any installation ratio.

If you have a racing buddy and want to run his setup, make sure you know the relationship of the motion ratios and spring angles when comparing the two cars. His motion ratios and spring angles may well be very different than yours and the setup may not translate well.

Rear Spring Installation In the rear for a solid axle suspension, the car "feels" the spring base as the distance between the top of the two springs. This is all the car knows and it is as if the chassis were sitting on a pair of springs that are resting on the ground. The rear solid axle assembly is a solid base for the springs to sit on and the car does not know, nor does it care, where the wheels are located.

Overturning moments acting through the center of gravity are effected by the resistance to roll created by the spring base width and spring split, the rear moment center height, and the rates of each spring. To effect changes to the rolling tendencies of the rear suspension, we need to look at altering the spring base (distance between the tops of the rear springs), the installed spring angle, and/or the moment center height. The narrower the spring base, the greater the tendency for the rear end to roll for a given set of springs and moment center height.

A narrow spring base can be a real problem for some types of cars. The four-link, Dirt Late Model cars sometimes have the springs installed at high angles with the top of the coilover positioned well inside the framerails. This severely limits what we can do to eliminate excessive rear roll in the car.

10/13A swing arm rear suspension is designed so that the spring is mounted onto the trailing arm and moves a shorter distance in relation to the wheel. This results in a motion ratio much like that found in the front suspension. The rate felt by the car is much less than the installed spring rate because, as the car rolls, the top and bottom of the spring moves. For every inch the chassis moves vertically, the spring compresses less. Finding the installed rate is just like finding wheel rate at the front. You have a motion ratio combined with a spring angle.arm

Many teams have moved the top shock/spring mounts out closer to the wheel to increase the rear spring base. The swing-arm types of dirt car have a very wide spring base by virtue of being mounted straight up, but suffer from another variable-motion ratio. The spring is mounted directly on the trailing arm, similar to the front spring mount, and that reduces the rate that the car "feels."

We can do a calculation similar to the front wheel rate calculation to find the effective spring rate. In most cases, we see about half the effective rate as the installed spring rate. So, if we install, say, a 200-pound spring in the RR, the car will react as if there was a 100-pound spring mounted directly to the rear end.

Stock classes, as well as sedan touring cars based on stock dimensions such as the USAR Pro Cup, Camping World Trucks, Nationwide Series, and Sprint Cup Series all suffer from a too-narrow rear spring base. To overcome this deficiency, some teams have deviated from conventional spring rate layouts and gone to a stiff RR spring. For this very reason, the BBSS setups tend to favor these cars. This "excess" spring split greatly reduces the roll tendencies of the rear suspension and caused a more balanced setup in the car, especially when using a large diameter sway bar.

Now, we see this same method used in many more series combining softer front springs with a higher rear moment center (Panhard bar). What these teams have done is work out a solution to their balance problems associated with the narrow rear spring base. The key objective still remains to balance the roll tendencies of the two suspension systems.

Make sure that you know your true spring rates, replace bent or fatigued springs, and always know your wheel rates when altering the mounting points or transferring the setup from one car to another. The car basically rides on four springs and the more we know about how they affect the setup in our cars, the more accurately we can develop a winning setup.